J. Nannoplankton Res. 30 (1), 2008, pp.23-33 © 2008 International Nannoplankton Association, Inc.
ISSN 1210-8049 Printed by Cambridge University Press, UK

23

A new Eocene Chiasmolithus species: hypotheticalreconstruction of its phyletic lineage
Davide Persico*, Giuliana Villa
Dipartimento di Scienze della Terra, Università degli Studi di Parma, Viale GP Usberti, 157A, 43100 Parma, Italy; *davide.persico@unipr.it

Manuscript received 31st July, 2007; revised manuscript accepted 18th March, 2008

Abstract Chiasmolithus is abundant in Southern Ocean Ocean Drilling Program Sites 689, 738 and 748, and consequently provides an opportunity to study, in detail, evolutionary lineages and investigate the biostratigraphic distribution of some of the species. In particular, in the Middle to Late Eocene, a new species, Chiasmolithus eoaltus, hasbeen recognised and distinguished from Chiasmolithus altus. Quantitative analysis highlights a gap between the last
occurrence of C. eoaltus in the Upper Eocene and the first occurrence of C. altus in the Lower Oligocene. The morphological similarity is probably the result of convergent evolution. Additionally, the evolutionary steps that mark thephylogenesis of Chiasmolithus species during the Middle Eocene to Late Oligocene were investigated. Biostratigraphic
and morphometric data permit reconstruction of a hypothetical phylogenetic tree, delineated by speciation events: wehypothesise that Chiasmolithus solitus gave rise to C. oamaruensis and C. eoaltus between 40.69 and 38.34Ma (Reticulofenestra reticulata Nannofossil Zone), followed by the evolution of C. altus from C. oamaruensis between 33.97
and 32.49Ma (Blackites spinosus Nannofossil Zone).
A correlation between palaeoclimatic interpretations and the Chiasmolithus evolutionary tree underline that sea-surface temperature changes played a key role in the nannofossil evolutionary process, and suggest that phyletic gradualism, a recognised theoretical approach to evolutionary process, resulted from intraspecific variability subordinatedto natural selection. This study represents, beyond the description of a new nannofossil species, confirmation of the
neo-Darwinian concept of phyletic gradualism.

Keywords Calcareous nannofossils, Chiasmolithus, Eocene, Oligocene, evolution, phyletic gradualism, Southern Ocean

Kaenel & Villa, 1996). The different ages may arise fromtaxonomic problems, specifically, from the differing biostratigraphical distributions of species that have not yet been
0°
°
-30

°

689

°

-60

°

30

60

Maud Rise

748
-90°
0°

90°

Kerguelen
738 Plateau

0°

-12

12

15

0°

-15

0°

180°

1. Introduction
The genus Chiasmolithus was erected by Hay et al. (1966)
to include elliptical placoliths with a large central-opening,
spanned by an X-shaped central-structure made of fourarms. Gartner (1970) studied the species of the genus inorder to determine phyletic relationships on the basis of thecentral-area structures. He determined the characteristics
that distinguish the species: placolith size, number of elements in the distal shield, curvature of the elements, size ofthe central opening, shape of the cross-bars, presence/absence of accessory structures, and structure of the crossbars. With these criteria, Gartner (1970) divided the genus
into two groups: species with cross-bars made of calcite
laths, and species with cross-bars composed of tabular calcite prisms. Perch-Nielsen (1985), similarly to Gartner(1970), separated the genus into two subgroups, and included Chiasmolithus altus in the group with cross-bars
made of calcite laths.
Several quantitative evolutionary studies have been performed on the genus Chiasmolithus (Gartner, 1970;
Romein, 1979; Perch-Nielsen, 1985), but only Firth &Wise (1992) proposed a phylogenetic lineage for C. altus,
suggesting a link from Chiasmolithus expansus to C. oamaruensis to C. altus.
The evolutionary first occurrence (FO) of C. altus has
been a matter for discussion among several authors, the age
reported varying from the Middle Eocene (Firth & Wise,
1992) to the Early Oligocene (Perch-Nielsen, 1985; de

km
0

1000

Figure 1: Location map of studied sites (modified after Villa & Persico,2006). Leg 113, Hole 689D - 64˚31.009’S/3˚06.026’E; Leg 119, Hole
738B - 62˚43’S/82˚47’E; Leg 120, Hole 748B - 58˚26.45’S/78˚58.89’E

24Persico, Villa

Figure 2: Stratigraphic ranges of the involved species, shown against ages of sampled levels (Persico & Villa,
2004; Villa et al., subm.). Nannofossil biozonation after Wei & Thierstein (1991), with basal boundaries of the
Discoaster saipanensis and Chiasmolithus oamaruensis Zones modified using new biostratigraphic datums(LCO C. solitus and FCO C. oamaruensis, respectively) from Site 748 (Persico & Villa, 2004; Villa et al., subm.)

identified as distinct from one another.
Biometric studies have been reported by Backman(1980), Backman & Hermelin (1986) and Pujos (1987),
concerning the taxonomic and evolutionary links between
species of Coccolithus and Reticulofenestra. Recent studies have used a biometric approach to highlight relationships between the abundances of some morphotypes andparticular palaeoenvironmental conditions (Bauman, 1993;
Bollman, 1997; Knappertsbusch et al., 1997; Henderiks &Renaud, 2001). Generally, these studies have focused on
species with fairly simple morphologies, that is, ellipticalplacoliths with a central
perforation. In this paper,
we study taxa with more
complexmorphology,
where an X-shaped centralstructure is the main changing taxonomic feature.
Previous
studies(Perch-Nielsen, 1985; Firth& Wise, 1992) have underlined that the variable, highevolutionary rate of themorphologically heterogeneous genus Chiasmolithus
is represented by an elevated number of FOs, 21 in
the Palaeogene, of which
15 are in the Eocene(Perch-Nielsen,1985),
recognised in northern tosouthernhigh-latitudes(Firth & Wise, 1992).
These widespread geographic distributions, coupled with high intraspecificvariability, suggest that
ecological pressures havelikely generated populations that differ slightly
from each other. A population is defined as a group of
specimens belonging to thesame species, and distributed in a defined geographic area (Odum, 1971).
A population is characterised by intraspecificvariability, within whichmorphotypes can be recognised. If such morphotypes
can be determined to havediscretestratigraphic
ranges, they may be usefulas biostratigraphic tools. If
isolation of the populationis persistent, distinct new

species can originate.
Common to abundant, moderately well-preserved specimens of Chiasmolithus occur in Middle Eocene-Oligocene
Southern Ocean sediments recovered at Ocean Drilling
Program (ODP) Sites 738 (Florindo et al., subm), 748(Villa & Persico, 2006; Villa et al., subm.) and 689 (Persico& Villa, 2004; Figure 1). These sites provide a good opportunity to study, in detail, morphology, temporal distribution, and the phyletic lineage of certain Chiasmolithus
species.
Furthermore, at ODP Sites 738, 748 and 689, in an in-New Eocene Chiasmolithus...phyletic lineage

terval representing part of the Middle-Late Eocene, we detected specimens with morphological features similar to
those of C. altus, but with subtle differences in morphology
and stratigraphic range, suggesting a distinct new species.
Consequently, we performed a detailed study on the biometry of the C. solitus-oamaruensis-altus group, to determine the real stratigraphic distribution of C. altus, to verify
the existence of a new species, and to suggest phylogenetic
relationships between C. altus, C. oamaruensis, C. solitus
and C. expansus.

25

C. altus-like specimens, while only morphological observations were made for C. oamaruensis. This is because of
the latter’s easily recognisable morphology, despite its highintraspecific variability. These three defined taxa were chosen because of the stratigraphic continuity between theirranges, which was essential for hypothesising phylogenetic
relationships. Chiasmolithus expansus was excluded because it has a discontinuous distribution in the studied interval.
Micrographs of 90 specimens per species were taken,
using a digital video camera (Scion Fire-wire Camera,
Model CFW-1612C) mounted on a Zeiss Axioscope transmitting LM. The high-quality images were then measured
with Scion Image software, written by Dr. Jeremy Young(Young et al., 1996).
The recorded biometric parameters (shown in Figure 3)
are: size (placolith length, L; placolith width, l), morphology of the central-area (central-opening length, D; centralopening width, d), and size of the central part of the cross(length of the central part of the cross, a; width of the central part of the cross, b).
Frequency histograms of placolith length (L) and placolith width (l) are correlated with the dispersion diagramsof the same parameters (Figure 4). The dispersion diagramsof the biometric measurements were plotted, showing the
distribution of groups partly overlapping (Figure 5).

2. Material and methods
Sample material from ODP Sites 748, 738 and 689 (Figure1) was used for this study. Calcareous nannofossils fromthese sites were originally studied by Wei & Wise (1990)
and Wei & Thierstein (1991), both at low resolution. Highresolution, quantitative analyses were later performed by
Persico & Villa (2004), Villa &Persico (2006) and Villa etal. (subm.). For this work, we used the same slides as wereused for these previous studies, prepared using the settlingtechnique described by de Kaenel & Villa (1996), at 10cm
intervals from each core. All samples analysed were froma fixed quantity of sediment (2mm3). Species abundanceswere counted in the light microscope (LM) at 1250x magnification.
Samples were selected firstly on the basis of the abundance of Chiasmolithus spp., and secondly on best preservation (specimens with complete central cross-bars). The 3. Results and discussion
recorded poor preservation of nannofossils at the Eocene/ 3.1 Biometric results
Oligocene boundary is the reason for the lack of studied Biometric analysis of Chiasmolithus solitus has underlined
samples in this time-interval (Figure 2). To compensate for the small size and easily identifiable morphology of thethis, we used a mixture of samples
from three areas in the SouthernD
Ocean, to avoid the influence ofL
local variability on the morphometric data.
The primary aim of this studyba
was to apply the precise morphologl dical values of Chiasmolithus altus to
specimens with an altus-like morphology from the Middle-Latebiometric measurements (µm)
Eocene, in order to prove that certain
specimens differed enough from C.
SPECIESLDlda/b
altus sensu stricto to constitute a
new species, so that we could ulti10.9
C. altus
13
6.7
4.6
1.1
mately provide a more precise age
for the FO of C. altus. In order toa=bachieve this, we studied two disparate stratigraphic intervals, in
C. altus-like
14.1
7.6
11.8
5.4
1.3
which the two forms were believed(from previous, non-biometric studa>bies) not to overlap, that is, the Middle-Upper Eocene (five samples) andthe Lower-Upper Oligocene (three
C. solitus
10.5
5.3
8.8
3.9
1.8
samples; Figure 2).
a>bBiometric data was collected on
Chiasmolithus solitus, C. altus and Figure 3: Biometric parameters and corresponding mean values of studied species of Chiasmolithus
b

a

b

a

b

a

26

Persico, Villa

Figure 4: Distribution of placolith length and width of all Chiasmolithus altus, C. altus-like (eoaltus) and C. solitus specimens, measured from Middle to Late Oligocene samples. A) Placolith length
vs. width. B) Histogram of placolith length frequency; C) Histogram of placolith width frequency.
Top right: Placolith schematics illustrating morphometric measurements of length (L) and width (l)cross-bars, which can vary from one
bar being sinusoidal to two arms
being non-aligned (Figure 4), as distinctive characteristics.
Figure 3 shows a comparison
between the mean biometric valuesof the Chiasmolithus altus, C. altuslike and C. solitus specimens. The
Middle to Upper Eocene C. altuslike specimens have biometric meanvalues of L = 14.1μm, l = 11.8μm,
with d = 5.4μm and D = 7.6μm (Figure 3). The central cross is both thinner and larger than in the Oligocene
C. altus specimens, within a large
central-opening (d/l = 0.46). The
central-area width (d), equivalent tohalf l, is an easily recognisable characteristic in the LM, therefore a distinctive
taxonomical
feature.
Generally, the frequency histograms
show a unimodal distribution ofthese features in the population. The
L histogram of C. altus-like specimens highlights an imperfect unimodal distribution, which probably
results from the poor preservation ofsome distal rims (Figure 4).
The specimens of C. altus s.s.
from the Middle to Upper Oligocene(ODP Site 748) samples have meanvalues of L = 13.00μm and l =10.9μm, with a d = 4.6μm. The ratio
d/l = 0.42 defines a central area proportionally smaller, with respect tothe same parameter for the Middle
Eocene specimens (d/l = 0.46). In
order to facilitate comparison between the numerical data in the biometric analyses, reciprocal values(l/L, d/D and a/b) have been compared (Figure 5). The dispersion diagrams obtained show three areas,
partially overlapping, the limits ofwhich identify the morphometriccharacteristics of every species. In
the l/L diagram (Figure 5A), the C.
solitus group is quite separated fromthe other two taxa. These latter areassembled in two groups that partially overlap. This probably explains, with the overlapped
distributions of the three species inthe d/D diagram (Figure 5B), the reason for the lack of taxonomical separation of C. altus from C. altus-likespecimens and consequent biostrati-

New Eocene Chiasmolithus...phyletic lineage

graphical confusion over the FO of C. altus. The mean values of the a/b data (Figure 5C) for the three taxa highlightthe presence of three distinct groups, characterised by having different cross-bar morphology. The reciprocal comparison of the absolute measurements in the three species
was made possible using the ratio a/b.
The mean value of these ratios are representative of the morphology of thecentral cross with a/b = 1.1 in C. altus(aligned arms), a/b = 1.3 in C. altus-like(two shifted arms), and a/b = 1.8 in C.
solitus (two very shifted arms; Figure
3).
The three dispersion diagrams (Figure 5) demonstrate a morphologicalcontinuity between the three morphologies. However, the biostratigraphic distributions underline an evolutionary
discontinuity between them. On thebasis of the biostratigraphic discontinuity between C. altus-like forms and C.
altus, and the different biometric measurements of the three species, we propose the erection of a new species,
Chiasmolithus eoaltus, for the taxon
with a distinct Middle-Upper Eocene
distribution (see below).
3.2 Palaeoclimatic
variations and evolution
The alternation of cold and warm climatic phases that preceded the Eocene/
Oligocene boundary is reflected in nannofossil assemblage variations (Persico& Villa, 2004). Late Eocene sea-surface
temperature (SST) fluctuations, triggered by palaeoceanographic changes,
played a key role in speciation events
that occurred relatively rapidly at different latitudes.
Particularly, SST changes, recognised on the basis of calcareous nannofossils from the Middle Eocene to theLate Oligocene (Villa et al., subm.), underline the palaeoecological relationshipbetween morphological variation within
the genus Chiasmolithus and climaticchange (Figure 6). Chiasmolithus has
been hypothesised as a cool-water taxon(Wei & Wise, 1990; Bralower, 2002;
Persico & Villa, 2004). Firth & Wise
(1992) suggested that Chiasmolithus
altus is a cold-water taxon, while C. oamaruensis is a cool-water taxon. At
ODP Site 748, quantitative analysis allowed a more precise definition of the
distribution and abundance patterns of27

Chiasmolithus, and construction of a temperate-/warmwater taxa index (TWWT: Villa et al., subm.). This index,
plotted against this Chiasmolithus distribution data, suggests a link between species morphology and SST conditions.

Figure 5: Distribution plots comparing the six morphological parameters measured from all specimens: A) l/L; B) d/D; C) a/b

Persico, Villa
28

NP19-20

NP21-22NP23

NP24-25

Martini(1971)

CP15a

CP15b

CP16

CP17-18

CP19

Okada &
Bukry
(1980)

NP17 CP14b

NP18
CP14a

Epoch

10

12

13

14

15

16

17

18

19

Nannofossil zone
Wei & Thierstein (1991)
Reticulofenestra bisecta

Chiasmolithus altus

Reticulofenestra daviesii
Blackites spinosus

Reticulofenestra oamaruensis
Isthmolithus recurvus

Chiasmolithus oamaruensis

Discoaster saipanensis

Reticulofenestra reticulata

Additional nannofossil Palaeoclimaticinterpretation
event
Nannofossil event mod. aftert+w/(t+w+c)*100
50
100
Wei & Thierstein (1991) (from Villa et al., subm.) 0*

LO C. altus

LO R. umbilica
FO C. altus

LO C. oamaruensis
FO C. abisectus

LO R. clatrata

LCO
Discoaster spp.

FO C. oamaruensis
FO C. eoaltus
LCO C. formosus

FO I. recurvus
LO C. solitus

FO R. bisecta

LO N. dubius

LO R. reticulata
LO C. eoaltus

LO I. recurvus
LO R. oamaruensis
FO R. oamaruensis
FCO I. recurvus

LCO C. solitus

FCO C. oamaruensis

FO R. reticulata

Hypothetical evolutionary lineage(this work)

C. oamaruensis - C. altus
trend of morphological changes

C. solitus - C. oamaruensis
trend of morphological changes

Chiasmolithus altus

70

80

90

100

110

120

130

140

150

160

170

Reticulofenestra umbilicus

C. solitus - C. eoaltus
trend of morphological changes

Chiasmolithus eoaltus

Site 748core
20

Cool interval

Chiasmolithus solitus

Depth(mbsf)

Figure 6: Biostratigraphic context and correlation between palaeoclimatic interpretations and hypothetical evolutionary lineage, incorporating hypothetical morphological changes in the cross-bars through time.
FO - first occurrence, LO - last occurrence, FCO - first common occurrence, LCO - last common occurrence

Warm interval

Chiasmolithus oamaruensis

Late Oligocene
Early Oligocene

Late Eocene

Middle Eocene* Temperate-/warm-water taxa index

NP16

New Eocene Chiasmolithus...phyletic lineage

Chiasmolithus solitus goes extinct at a level of transition
from warm- to cool-water conditions in the Middle Eocene,at the beginning of a cooling trend, which culminates at the
Eocene/Oligocene boundary (Figure 6). Therefore, it may
represent a warm-water morphology, within a cool-water
genus. SST fluctuations thus may be the reason for the speciation process that resulted in the evolution of C. oamaruensis and C. eoaltus sp. nov.
C. oamaruensis lived from the late Middle Eocene to
Early Oligocene. During this time, the TWWT index showsa general cooling trend, punctuated by several warming
pulses. The persistence of this species through these cooling/warming episodes, and the elevated intraspecific variability occurring during its range, suggest that themorphological variability is derived from genetic recombination, linked to palaeoenvironmental changes, and thus
C. oamaruensis may represent a transitional morphology,
restricted to periods of fluctuating SSTs.
The extinction of C. solitus is preceded by the appearance of C. eoaltus sp. nov., interpreted as a cool-SST morphological adaptation, ranging from the Middle to Late
Eocene, during an interval characterised by a continuous
cooling trend. This trend is followed by a phase ofcool/temperate alternation, with re-establishment of temperateconditions
immediately
beforethe
Eocene/Oligocene boundary transition. We hypothesisethat this palaeoenvironmental situation most likely caused
the extinction of this species.
C. altus is interpreted as a cool-SST form. It originatedin the Early Oligocene, during a phase of persisting coolSST conditions, just after the Oi-1 event (Miller et al.,1987), and went extinct at the base of the warming phasein the latest Oligocene.
There is no biostratigraphical continuity between C.
eoaltus sp. nov. and C. altus. This implies that there is no
direct phylogenetic relationship between the forms and,
consequently, we suggest these forms are an example ofevolutionary convergence, linked to cooling events that occurred at different times.

29

from C. oamaruensis (Figure 6).
We hypothesise that these evolutionary inceptions andrelationships in Chiasmolithus are related to SST changes.
C. solitus (Middle Eocene) represents a warm-water form,
and thus last occurs at the beginning of a cooling trend.
SST fluctuations most likely caused speciation from C.
solitus, giving rise to C. oamaruensis and C. eoaltus sp.
nov. We exclude, for biostratigraphic reasons, Chiasmolithus medius as a possible ancestor, because it goes extinct at the top of NP15 (Middle Eocene; Perch-Nielsen,1985), while C. eoaltus sp. nov. and C. oamaruensis bothhave their FOs in the upper part of NP16 (Middle Eocene).
C. oamaruensis (Middle Eocene-Early Oligocene) represents a palaeoecological transitional form, characterisedby high intraspecific variability, probably deriving from genetic recombination, linked to palaeoenvironmental
change.
C. eoaltus sp. nov. (Middle to Late Eocene) is interpreted as a cool-SST form, representing a short phase ofcontinuous cooling. Its extinction is linked to a phase offluctuating SST cooling/warming and the temporary returnof temperate conditions immediately preceding the
Eocene/Oligocene boundary. C. altus (Early-Late
Oligocene) is interpreted as a cool-temperature form, originating during stable, cool-SST conditions. The similar, yet
distinct, morphologies of C. eoaltus sp. nov. and C. altus
are thus an example of evolutionary convergence linked tocooling events.
The overlapping biometric data are a response to highintraspecific variability and underline the gradual evolutionary transition of some Chiasmolithus species. Stratigraphic sequences characterised by high sedimentation
rates, continuous successions, and well-preserved assemblages, such as the Eocene-Oligocene records from Southern Ocean ODP sites, represent ideal conditions forevolutionary studies. Our data suggest a phyletic gradualism in the Chiasmolithus species, which remains indistinct
in discontinuous and scarce stratigraphic records.
Taxonomical appendix

4. Conclusions
Biostratigraphic and biometric analyses have allowed thedifferentiation of Chiasmolithus altus (Early-Late
Oligocene) from a new species, C. eoaltus sp. nov., which
is characterised by a short stratigraphical range in the Middle-Late Eocene and by differences in the central cross,which is thinner and larger; the bars, which are not perfectly aligned at 90°; and the relative size of the centralopening (d/l), which is ~1/2 (0.46), compared to ~1/2.5(0.42) in C. altus.
The distinction between these taxa allows us to provide
more accurate FOs, and also suggests a different phylogenetic tree to those of Gartner (1970) and Firth & Wise
(1992), which were based on morphological analysis andbiostratigraphic data from a low-resolution sample suite.
In the new tree, Chiasmolithus solitus gave rise to C. oamaruensis and C. eoaltus sp. nov., with C. altus arising

Chiasmolithus eoaltus sp. nov.
Plate 1, figs 1-12; Plate 2 Figs 1-6
Derivation of name: From Eocene, referring to the distribution of the species, and altus, referring to the similarities
in morphology to C. altus.
Diagnosis: Very large, elliptical species of Chiasmolithus,
with a large central-opening surmounted by an X-shaped
structure consisting of four arms. The width of the central
opening is 0.5 of the coccolith’s width.
Description: Large, elliptical placolith characterised by anelliptical, and relatively large, central-opening, spanned bya delicate X-shaped structure, consisting of two straight andtwo curved arms, which meet about at 90˚, and which form
one straight and one sigmoidal bar. The length of the central part of the cross (a on Figure 3) varies from 1.6 toPersico, Villa

30

Plate 1
LM images of Chiasmolithus. XPL = cross-polarised light, PC = phase-contrast
XPL

PC

XPL

PC

1μm

Chiasmolithus eoaltus (same specimen)
Sample ODP 748B-17-6, 60-61cm
Middle Eocene

Chiasmolithus eoaltus (same specimen)
Sample ODP 748B-17-6, 60-61cm
Middle Eocene

XPL

XPL

PC

PC

Chiasmolithus eoaltus (same specimen)
Sample ODP 748B-17-6, 81-82cm
Middle Eocene

Chiasmolithus eoaltus (same specimen)
Sample ODP 738B-5-6, 78-80cm
Middle Eocene

XPL

XPL

PC

PC

Chiasmolithus eoaltus (same specimen)
Sample ODP 748B-17-6, 81-82cm
Middle Eocene

Chiasmolithus eoaltus (same specimen)
Sample ODP 748B-17-6, 81-82cm
Middle Eocene

XPL

XPL

PC

PC

Chiasmolithus altus (same specimen)
Sample ODP 748B-10-2, 15-16cm
Upper Oligocene

Chiasmolithus altus (same specimen)
Sample ODP 748B-10-2, 15-16cm
Upper Oligocene

PC

PC

PC

Chiasmolithus solitus (same specimen)
Sample ODP 748B-17-6, 60-61cm
Middle Eocene

PC

Chiasmolithus solitus (same specimen)
Sample ODP 748B-20-3, 16-18cm
Middle Eocene

New Eocene Chiasmolithus...phyletic lineage

31

Plate 2
SEM images of Chiasmolithus
proximaldistaldistal

9μm

10μm

9μm

Chiasmolithus eoaltus
Sample ODP 748B-17-6, 60-61cm, Middle Eocene
distalproximaldistal

8μm

10μm

10μm

Chiasmolithus eoaltus
Sample ODP 748B-17-6, 60-61cm, Middle Eocene
distalproximaldistal

8μm

9μm

8μm

Chiasmolithus altus
Sample ODP 748B-10-2, 15-16cm, Upper Oligocene
proximalproximalproximal

7μm

8μm

8μm

Chiasmolithus altus
Sample ODP 748B-10-2, 15-16cm, Upper Oligocene
distalproximal

8μm

Chiasmolithus solitus
Sample ODP 748B-20-3, 16-18cm, Middle Eocene

8μm

distal10μm

32Persico, Villa

2.9μm. Measured along the short axis, the central-area
opening is equal to one-half the total width of the coccolith. The rim is composed of about 80 elements and is faintin the outer portion in cross-polarised light.
Differentiation: C. eoaltus differs from C. altus in having
a larger central-opening, a thinner rim, and a cross-bar
structure not perfectly aligned (one bar is slightly sigmoidal). Compared to C. medius (length 7.7-8.6μm), C.
eoaltus (11-18μm long) is larger, and the almost-symmetrical central-area structure is essentially composed of one
straight bar and one slightly sigmoidal bar. The centralopenings of C. medius and C. solitus are spanned by a pronounced sigmoidal bar. Also, the stratigraphic range of C.
medius does not overlap with the range of C. eoaltus. C.
eoaltus is distinguished from C. expansus (15-23μm) by its
smaller size, and by the lack of the distal central-area crest.
C. expansus is characterised by a large central-opening,
spanned by symmetrical, hemispherical, X-shaped crossbars (not sigmoidal).
Holotype: Pl.1, figs 1, 2 (slide and image curated in the
University of Parma).
Length: 11 to 18μm.
Type level: Sample ODP Leg 120, Hole 748B-17-6, 6061cm, Middle Eocene.
Type locality: ODP Hole 748B, Kerguelen Plateau, Southern Ocean.
Occurrence: C. eoaltus was recorded from Southern
Ocean Middle to Upper Eocene sediments, where it co-occurs with C. oamaruensis.
Range: Restricted to NP16 (upper part) to NP18 (upperpart), Middle to Upper Eocene.
Acknowledgements
Elisabetta Erba and Bianca de Bernardi are thanked for the useful suggestions during the biometric analyses, and the Earth Science Department of the University of Milan for the technicalsupport. We thank Eric de Kaenel, an anonymous reviewer andJackie Lees for their valuable comments, which greatly improved
the manuscript. In particular, Jackie Lees is acknowledged for her
linguistic and editorial remarks.

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